Oxygen-redox reactions in LiCoO2 cathode without O–O bonding during charge-discharge

Abstract

•Oxygen redox is active in highly charged LiCoO2 and largely reversible•O–O distance is shortened to ∼2.5 Å in highly delithiated LiCoO2 (85% Li extraction)•No O–O bond is formed in highly delithiated LiCoO2 LiCoO2 is a widely used cathode material for lithium-ion batteries with high volumetric energy density and tap density. Traditionally, only about 50% of the total lithium is utilized because over 50% Li extraction is believed to cause irreversible reactions and structural degradation with oxygen release. However, there is always a great interest in going beyond x = 0.5 in Li1−xCoO2 to increase the energy density. Therefore, a clarification of the controversial issues related to oxygen activities in highly delithiated LiCoO2 becomes critical. We use X-ray spectroscopy, X-ray and neutron scattering, as well as theoretical methods to study the nature of oxygen activity in LiCoO2 and reveal that oxygen oxidation takes place globally in the lattice without formation of O–O bonds, suggesting little or no oxygen release from the bulk. This provides the rationality for achieving a reversible deep delithiation in LiCoO2. Oxygen activity in highly delithiated LiCoO2 is critical to fully utilizing the energy density of this high-tap-density cathode but still lacks a clear understanding. In this work, we combined the results of several experimental techniques, especially resonant inelastic X-ray scattering (RIXS) and neutron pair distribution function (NPDF) analysis, together with theoretical calculations to study this topic. Our results conclude that oxygen redox takes place globally in the lattice, rather than forming localized dimerization as previously thought. RIXS results directly reveal the reversible oxygen redox, and NPDF results show that the O–O pair distance is considerably shortened in the highly delithiated LiCoO2. Theoretical calculations indicate that no O–O bonding is formed in LiCoO2, in sharp contrast to the lithium-rich system in which O–O bonding does form. These results provide the rationale for achieving a reversible deep delithiation and high energy density for LiCoO2-based electrodes. Oxygen activity in highly delithiated LiCoO2 is critical to fully utilizing the energy density of this high-tap-density cathode but still lacks a clear understanding. In this work, we combined the results of several experimental techniques, especially resonant inelastic X-ray scattering (RIXS) and neutron pair distribution function (NPDF) analysis, together with theoretical calculations to study this topic. Our results conclude that oxygen redox takes place globally in the lattice, rather than forming localized dimerization as previously thought. RIXS results directly reveal the reversible oxygen redox, and NPDF results show that the O–O pair distance is considerably shortened in the highly delithiated LiCoO2. Theoretical calculations indicate that no O–O bonding is formed in LiCoO2, in sharp contrast to the lithium-rich system in which O–O bonding does form. These results provide the rationale for achieving a reversible deep delithiation and high energy density for LiCoO2-based electrodes. LiCoO2 was proposed as a cathode material for lithium-ion batteries (LIB) in 19801Mizushima K. Jones P.C. Wiseman P.J. Goodenough J.B. LixCoO2 ((0<x<-1): a new cathode material for batteries of high energy density.Mater. Res. Bull. 1980; 15: 783-789Crossref Scopus (2680) Google Scholar and used in the first LIB released by SONY in 1991.2Nagaura T. Tozawa K. Progress in Batteries Solar Cells. Volume 9. 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Previous studies suggested that oxygen can be involved in the redox reaction in highly charged LiCoO2,8Ceder G. Chiang Y.-M. Sadoway D.R. Aydinol M.K. Jang Y.-I. Huang B. Identification of cathode materials for lithium batteries guided by first-principles calculations.Nature. 1998; 392: 694-696Crossref Scopus (686) Google Scholar, 9Van der Ven A. Aydinol M.K. Ceder G. Kresse G. Hafner J. First-principles investigation of phase stability in LixCoO2.Phys. Rev. B. 1998; 58: 2975-2987Crossref Scopus (559) Google Scholar, 10Yoon W.-S. Kim K.-B. Kim M.-G. Lee M.-K. Shin H.-J. Lee J.-M. et al.Oxygen contribution on Li-Ion intercalation−deintercalation in LiCoO2 investigated by O K-Edge and Co L-edge X-ray absorption spectroscopy.J. Phys. Chem. B. 2002; 106: 2526-2532Crossref Scopus (246) Google Scholar, 11Mizokawa T. Wakisaka Y. Sudayama T. Iwai C. Miyoshi K. Takeuchi J. et al.Role of oxygen holes in LixCoO2 revealed by soft X-ray spectroscopy.Phys. Rev. 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Wu J. Csernica P.M. Takacs C.J. Nordlund D. Sun C.-J. et al.Metal-oxygen decoordination stabilizes anion redox in Li-rich oxides.Nat. Mater. 2019; 18: 256-265Crossref PubMed Scopus (162) Google Scholar Another work by House et al. on ARR in sodium cathode material revealed the correlation between local structural ordering, trapped O2 molecules, and oxygen vacancy.18House R.A. Maitra U. Pérez-Osorio M.A. Lozano J.G. Jin L. Somerville J.W. Duda L.C. Nag A. Walters A. Zhou K.-J. et al.Superstructure control of first-cycle voltage hysteresis in oxygen-redox cathodes.Nature. 2020; 577: 502-508Crossref PubMed Scopus (179) Google Scholar Tarascon and co-workers studied the 4d and 5d transition-metal oxide systems, such as Li2Ru1–xSnxO314Saubanère M. McCalla E. Tarascon J.-M. Doublet M.-L. The intriguing question of anionic redox in high-energy density cathodes for Li-ion batteries.Energy Environ. Sci. 2016; 9: 984-991Crossref Google Scholar,19Sathiya M. Rousse G. Ramesha K. Laisa C.P. 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Van Tendeloo G. et al.Visualization of O-O peroxo-like dimers in high-capacity layered oxides for Li-ion batteries.Science. 2015; 350: 1516-1521Crossref PubMed Scopus (476) Google Scholar In additional to experimental efforts, several theoretical studies have also been carried out by different groups to gain insight on key factors that determine the nature of ARR in transition-metal oxides. For example, Seo, Ceder, and co-workers15Seo D.H. Lee J. Urban A. Malik R. Kang S. Ceder G. The structural and chemical origin of the oxygen redox activity in layered and cation-disordered Li-excess cathode materials.Nat. Chem. 2016; 8: 692-697Crossref PubMed Scopus (654) Google Scholar used first-principle calculations to show that the presence of nontransition metals (like Sn) is favorable for dimer formation as the less directional Sn–O bond can facilitate the displacement of oxygen ions, which is considered as a necessary condition for forming O–O bonds. They also pointed out that oxygen anions are activated when there is Li–O–Li atomic configuration, suggesting the necessity of a Li-excess local environment around oxygen. Also, by using first-principle calculations, Saubanere et al. identified the key role of covalency in forming O–O dimers.14Saubanère M. McCalla E. Tarascon J.-M. Doublet M.-L. The intriguing question of anionic redox in high-energy density cathodes for Li-ion batteries.Energy Environ. Sci. 2016; 9: 984-991Crossref Google Scholar Their results suggested that a strong covalency between transition metal (TM) and oxygen can stabilize the oxygen holes generated by the oxygen reduction and establish a reversible redox couple between O2− and peroxo-like species (O2)n− which is likely to result in good electrochemical performance. In contrast, a weak covalency would lead to de-coordination of (O2)n− species and ultimate oxygen release as reported by Chen and Islam22Chen H. Islam M.S. Lithium extraction mechanism in Li-Rich Li2MnO3 involving oxygen hole formation and dimerization.Chem. Mater. 2016; 28: 6656-6663Crossref Scopus (154) Google Scholar in their recent paper. This oxygen release is the origin of poor electrochemical performance, in particular the serious voltage-fading issue.23Hu E. Yu X. Lin R. Bi X. Lu J. Bak S. Nam K.-W. Xin H.L. Jaye C. Fischer D.A. et al.Evolution of redox couples in Li- and Mn-rich cathode materials and mitigation of voltage fade by reducing oxygen release.Nat. Energy. 2018; 3: 690-698Crossref Scopus (378) Google Scholar Although O–O bonding is typically considered to be the chemical origin of oxygen-redox reactions in most studies, Luo and Bruce et al. reported that oxygen holes are localized without forming peroxo-type O–O bonding during the ARR of Li1.2Ni0.13Co0.13Mn0.54O2.24Luo K. Roberts M.R. Hao R. Guerrini N. Pickup D.M. Liu Y.S. Edström K. Guo J. Chadwick A.V. Duda L.C. Bruce P.G. Charge-compensation in 3d-transition-metal-oxide intercalation cathodes through the generation of localized electron holes on oxygen.Nat. Chem. 2016; 8: 684-691Crossref PubMed Scopus (616) Google Scholar The chemistry of oxidized oxygen states and the debates on O–O bonding formation are critical to the practical utilization of ARR in battery applications. Unfortunately, the direct probe of ARR states in LiCoO2 remains elusive, and debates regarding whether or not the O–O peroxo-dimer is formed in 3d transition-metal-based electrodes remain inconclusive. To answer such an important question, a combination of state-of-the-art characterization techniques is required to probe the lattice structure, electronic structure, and chemical bonding. In addition, detailed data analysis and theoretical calculations are also needed. In this work, X-ray spectroscopy, neutron scattering, and theoretical calculations are used to study the ARR in LiCoO2 and the possibility of O–O bonding in it. First, the direct detection of the oxygen-redox states in LiCoO2 is provided using the high-efficiency mapping of resonant inelastic X-ray scattering (mRIXS),25Yang W. Devereaux T.P. Anionic and cationic redox and interfaces in batteries: advances from soft X-ray absorption spectroscopy to resonant inelastic scattering.J. Power Sources. 2018; 389: 188-197Crossref Scopus (126) Google Scholar,26Dai K. Wu J. Zhuo Z. Li Q. Sallis S. Mao J. Ai G. Sun C. Li Z. Gent W.E. et al.High reversibility of lattice oxygen redox quantified by direct bulk probes of both anionic and cationic redox reactions.Joule. 2019; 3: 518-541Abstract Full Text Full Text PDF Scopus (134) Google Scholar which shows experimental evidence of oxygen oxidation in high-voltage charged LiCoO2. Second, we employ neutron pair distribution function (NPDF) analysis to reveal the details of the oxygen-related structural changes. The effectiveness and uniqueness of NPDF in studying ARR have been successfully demonstrated.27Rong X. Hu E. Lu Y. Meng F. Zhao C. Wang X. Zhang Q. Yu X. Gu L. Hu Y.-S. et al.Anionic redox reaction-induced high-capacity and low-strain cathode with suppressed phase transition.Joule. 2019; 3: 503-517Abstract Full Text Full Text PDF Scopus (122) Google Scholar,28Rong X. Liu J. Hu E. Liu Y. Wang Y. Wu J. Yu X. Page K. Hu Y.-S. Yang W. et al.Structure-induced reversible anionic redox activity in Na layered oxide cathode.Joule. 2018; 2: 125-140Abstract Full Text Full Text PDF Scopus (187) Google Scholar To the best of our knowledge, this is the first time that these two powerful techniques are combined for studying the ARR in LiCoO2. A striking finding is that, although we reveal clear signatures of lattice oxygen-redox reactions and the shortening of the interlayer O–O distance, the theoretical calculation shows that the O–O bond is not formed in highly delithiated LiCoO2 systems. Our combined experimental and theoretical results provide the fundamental rationality to stimulate efforts in optimizing and utilizing reversible ARR in LiCoO2 for improving its applicable energy density. The LiCoO2 material used for this study was synthesized by a solid-state reaction method. The phase purity and the morphology of the LiCoO2 were examined by X-ray diffraction (XRD) and scanning electron microscope (SEM), and the results are shown in Figure S1. The first charge profile of LiCoO2/Li half-cell with a cut-off voltage of 4.9 V at a current C-rate of 0.1C (27.4 mA/g) is displayed in Figure 1A. During the first charging process, previous reports indicate that no obvious oxygen gas release was detected even when LiCoO2 was charged to 5 V in propylene carbonate (PC) or ethylene carbonate (EC)/dimethyl carboante (DMC) electrolyte.29Wang H. Rus E. Sakuraba T. Kikuchi J. Kiya Y. Abruña H.D. CO2 and O2 evolution at high voltage cathode materials of Li-ion batteries: a differential electrochemical mass spectrometry study.Anal. Chem. 2014; 86: 6197-6201Crossref PubMed Scopus (70) Google Scholar To investigate the charge-compensation mechanism associated with lithium extraction, Co K-edge hard and O K-edge soft X-ray absorption spectroscopy (XAS) measurements were performed on LiCoO2 at different charge states. Figure 1B presents the Co K-edge XAS results. The X-ray absorption near edge structure (XANES) spectra do not show an evident entire shift upon lithium extraction from LiCoO2, and therefore it is not possible to determine the change of oxidation state solely by using the edge position. The changes in the edge shape, ascribed to the changes in bond length and covalency, can be seen in spectra of LiCoO2 charged up to 4.5 V. Upon further charging, no notable change on spectra can be observed. The O K-edge soft XAS spectra, which correspond to the O 2p unoccupied states strongly affected by the hybridization effects from transition metals,30de Groot F.M. Grioni M. Fuggle J.C. Ghijsen J. Sawatzky G.A. Petersen H. Oxygen 1s x-ray-absorption edges of transition-metal oxides.Phys. Rev. B Condens Matter. 1989; 40: 5715-5723Crossref PubMed Scopus (1005) Google Scholar are shown in Figure S2. It is worth mentioning that the soft XAS data at O K-edge was taken in partial electron yield (PEY) mode, which has been recognized to be more sensitive to surface regions of samples.31Liu D. Shadike Z. Lin R. Qian K. Li H. Li K. Wang S. Yu Q. Liu M. Ganapathy S. et al.Review of recent development of in situ/operando characterization techniques for lithium battery research.Adv. Mater. 2019; 31: e1806620Crossref PubMed Scopus (212) Google Scholar,32Yogi C. Takamatsu D. Yamanaka K. Arai H. Uchimoto Y. Kojima K. Watanabe I. Ohta T. Ogumi Z. 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Soc. 2004; 151: A246-A251Crossref Scopus (86) Google Scholar In our results, all spectra show relatively sharp “pre-edge” features, at about 530 eV for LiCoO2 and get broadened into multiple features in charged states. These features correspond to Co 3d states that are hybridized to O 2p. The broad hump at 535–545 eV corresponds to the O 2p band mixed with the Co 4s/p states. Although the changes of O K-XAS pre-edge features have been widely used in the battery field as evidence of oxygen redox, we have recently clarified that this is not a reliable approach because such pre-edge features are dominated by the changes of transition-metal 3d states that are hybridized to O 2p states.25Yang W. Devereaux T.P. Anionic and cationic redox and interfaces in batteries: advances from soft X-ray absorption spectroscopy to resonant inelastic scattering.J. Power Sources. 2018; 389: 188-197Crossref Scopus (126) Google Scholar,35Roychoudhury S. Qiao R. Zhuo Z. Li Q. Lyu Y. Kim J.H. Liu J. Lee E. Polzin B.J. Guo J. et al.Deciphering the oxygen absorption pre-edge: a caveat on its application for probing oxygen redox reactions in batteries.Energy Environ. Mater. 2020; https://doi.org/10.1002/eem2.12119Crossref Scopus (20) Google Scholar Indeed, the changes on the O K-edge XAS pre-edge in LiCoO2 upon delithiation levels have been clearly assigned to the changes of Co 3d configurations in previous works.11Mizokawa T. Wakisaka Y. Sudayama T. Iwai C. Miyoshi K. Takeuchi J. et al.Role of oxygen holes in LixCoO2 revealed by soft X-ray spectroscopy.Phys. Rev. Lett. 2013; 111: 056404Crossref PubMed Scopus (94) Google Scholar The broadened pre-edge caused by charging corresponds to the new hybridization states between the oxidized Co and oxygen. The multiple peaks are due to the splitting of different Co 3d states that have been discussed in both experiments and theory.11Mizokawa T. Wakisaka Y. Sudayama T. Iwai C. Miyoshi K. Takeuchi J. et al.Role of oxygen holes in LixCoO2 revealed by soft X-ray spectroscopy.Phys. Rev. Lett. 2013; 111: 056404Crossref PubMed Scopus (94) Google Scholar The overall enhanced intensity reflects the stronger Co–O hybridization upon charging. In particular, the low energy intensity of the charged electrodes is caused by the lowered Co 3d conduction states after Co oxidation. Therefore, O K-edge XAS has been clarified not to be a probe of oxygen-redox states in this case,35Roychoudhury S. Qiao R. Zhuo Z. Li Q. Lyu Y. Kim J.H. Liu J. Lee E. Polzin B.J. Guo J. et al.Deciphering the oxygen absorption pre-edge: a caveat on its application for probing oxygen redox reactions in batteries.Energy Environ. Mater. 2020; https://doi.org/10.1002/eem2.12119Crossref Scopus (20) Google Scholar and we focus on mRIXS in this work, which has been established as a reliable probe of the oxygen-redox states in various battery cathodes.25Yang W. Devereaux T.P. Anionic and cationic redox and interfaces in batteries: advances from soft X-ray absorption spectroscopy to resonant inelastic scattering.J. Power Sources. 2018; 389: 188-197Crossref Scopus (126) Google Scholar,36Zhuo Z. Pemmaraju C.D. Vinson J. Jia C. Moritz B. Lee I. Sallies S. Li Q. Wu J. Dai K. et al.Spectroscopic signature of oxidized oxygen states in peroxides.J. Phys. Chem. Lett. 2018; 9: 6378-6384Crossref PubMed Scopus (61) Google Scholar We therefore leave the results of the O K-edge XAS in Figure S2 and seek more reliable and direct evidence of the oxygen-redox states in LiCoO2 using mRIXS. Compared with conventional XAS, mRIXS provides a completely new dimension of information about the energy of the emitted photons at each excitation energy of the XAS process.25Yang W. Devereaux T.P. Anionic and cationic redox and interfaces in batteries: advances from soft X-ray absorption spectroscopy to resonant inelastic scattering.J. Power Sources. 2018; 389: 188-197Crossref Scopus (126) Google Scholar The realization of high-efficiency mRIXS systems has enabled powerful spectroscopy for studying energy materials.37Qiao R. Li Q. Zhuo Z. Sallis S. Fuchs O. Blum M. Weinhardt L. Heske C. Pepper J. Jones M. et al.High-efficiency in situ resonant inelastic x-ray scattering (iRIXS) endstation at the Advanced Light Source.Rev. Sci. Instrum. 2017; 88: 033106Crossref PubMed Scopus (82) Google Scholar A striking feature around 523.5 eV emission energy and 531 eV excitation energy in mRIXS has been established as a fingerprint of the lattice oxygen-redox states in both Li-ion and Na-ion battery electrodes.25Yang W. Devereaux T.P. Anionic and cationic redox and interfaces in batteries: advances from soft X-ray absorption spectroscopy to resonant inelastic scattering.J. Power Sources. 2018; 389: 188-197Crossref Scopus (126) Google Scholar,26Dai K. Wu J. Zhuo Z. Li Q. Sallis S. Mao J. Ai G. Sun C. Li Z. Gent W.E. et al.High reversibility of lattice oxygen redox quantified by direct bulk probes of both anionic and cationic redox reactions.Joule. 2019; 3: 518-541Abstract Full Text Full Text PDF Scopus (134) Google Scholar Figures 2A–2C show the O K-edge mRIXS of LiCoO2 at pristine, charged and discharged states, respectively. Overall, all samples display several packets of broad features around 525 eV emission energy with a horizontal pattern. These are features from the Co–O hybridization states, with Co 3d and Co 4s4p hybridization features below and above 535 eV incident excitation energies, respectively. The intensities of these features increase upon charging (Figure 2B) due to the enhanced Co–O hybridization in the highly oxidized state.57Wu J. Li Q. Sallis S. Zhuo Z. Gent W.E. Chueh W.C. Yan S. Chuang Y.-d. Yang W. Fingerprint oxygen redox reactions in batteries through high-efficiency mapping of resonant inelastic X-ray scattering.Condens. Matter. 2019; 4: 5Crossref Scopus (34) Google Scholar Furthermore, the Co 3d hybridization feature also shifts toward lower incident energy. This is due to the increase of Co valence in the charged state compared with the pristine/discharged states, which is consistent with the O K-edge XAS pre-edge variation reported previously.35Roychoudhury S. Qiao R. Zhuo Z. Li Q. Lyu Y. Kim J.H. Liu J. Lee E. Polzin B.J. Guo J. et al.Deciphering the oxygen absorption pre-edge: a caveat on its application for probing oxygen redox reactions in batteries.Energy Environ. Mater. 2020; https://doi.org/10.1002/eem2.12119Crossref Scopus (20) Google Scholar It is clear that, while the majority of the mRIXS intensity is around 525 eV emission energy (vertical axis), which is typical for transition-metal oxides and corresponds to the XAS signals, a striking mRIXS feature emerges at 523.5 eV emission energy in the 4.8 V charged electrode, in sharp contrast to the pristine sample where this feature is absent. The sharp mRIXS peak is from intraband excitations to unoccupied O 2p final states36Zhuo Z. Pemmaraju C.D. Vinson J. Jia C. Moritz B. Lee I. Sallies S. Li Q. Wu J. Dai K. et al.Spectroscopic signature of oxidized oxygen states in peroxides.J. Phys. Chem. Lett. 2018; 9: 6378-6384Crossref PubMed Scopus (61) Google Scholar which can only arise if O2− is oxidized because O2− itself has no such unoccupied 2p state at all. Therefore, this 523.5 eV feature has been identified as the characteristic peak for oxygen-redox states in many battery cathodes.16Gent W.E. Lim K. Liang Y. Li Q. Barnes T. Ahn S.J. Stone K.H. McIntire M. Hong J. Song J.H. et al.Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides.Nat. Commun. 2017; 8: 2091Crossref PubMed Scopus (292) Google Scholar,25Yang W. Devereaux T.P. Anionic and cationic redox and interfaces in batteries: advances from soft X-ray absorption spectroscopy to resonant inelastic scattering.J. Power Sources. 2018; 389: 188-197Crossref Scopus (126) Google Scholar,26Dai K. Wu J. Zhuo Z. Li Q. Sallis S. Mao J. Ai G. Sun C. Li Z. Gent W.E. et al.High reversibility of lattice oxygen redox quantified by direct bulk probes of both anionic and cationic redox reactions.Joule. 2019; 3: 518-541Abstract Full Text Full Text PDF Scopus (134) Google Scholar,36Zhuo Z. Pemmaraju C.D. Vinson J. Jia C. Moritz B. Lee I. Sallies S. Li Q. Wu J. Dai K. et al.Spectroscopic signature of oxidized oxygen states in peroxides.J. Phys. Chem. Lett. 2018; 9: 6378-6384Crossref PubMed Scopus (61) Google Scholar,38Xu J. Sun M. Qiao R. Renfrew S.E. Ma L. Wu T. Hwang S. Nordlund D. Su D. Amine K. et al.Elucidating anionic oxygen activity in lithium-rich layered oxides.Nat. Commun. 2018; 9: 947Crossref PubMed Scopus (177) Google Scholar It is important to note that the oxidized oxygen states revealed by mRIXS correspond to the lattice-oxygen activities, which is different from other irreversible oxygen reactions such as oxygen release and surface reactions.26Dai K. Wu J. Zhuo Z. Li Q. Sallis S. Mao J. Ai G. Sun C. Li Z. Gent W.E. et al.High reversibility of lattice oxygen redox quantified by direct bulk probes of both anionic and cationic redox reactions.Joule. 2019; 3: 518-541Abstract Full Text Full Text PDF Scopus (134) Google Scholar,39Yang W. Oxygen release and oxygen redox.Nat. Energy. 2018; 3: 619-620Crossref Scopus (28) Google Scholar More importantly, this mRIXS feature of oxidized lattice oxygen disappears completely if the electrode is discharged (Figure 2C), indicating the reversible nature of the lattice-oxygen activity in LiCoO2. Therefore, our mRIXS results provide clear evidence for the reversible lattice o